JPH09241010A - Production of silicon nitride - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide easily crushable Si3 N4 by adding siliceous powder of a specified average particle diameter to metallic Si powder and carrying out molding and heating in a nitriding atmosphere. SOLUTION: Siliceous powder of >=10μm, especially about 100-500μm average particle diameter is added by 1-20 pts.wt. (expressed in terms of SiO2 ) to 100 pts.wt. metallic Si powder of about <=75μm, especially <=45μm average particle diameter and they are mixed and kneaded with an aq. PVA soln. The resultant kneaded body is press-molded, dried by heating and heated in two steps to 1,000 deg.C at 200 deg.C/hr heating rate and to 1,450 deg.C at 10 deg.C/hr heating rate in an atmosphere of gaseous N2 or a mixture of gaseous N2 with inert gas to obtain the objective easily crushable Si3 N4 . Powder of about <=0.5mm particle diameter obtd. by crushing the Si3 N4 with a crusher has about >=1.5m<2> /g specific surface area.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing silicon nitride having improved pulverizability of a silicon nitride ingot produced by a direct nitriding method of metal silicon (hereinafter referred to as "easy pulverization"). The object of the present invention is to provide inexpensive silicon nitride powder as a raw material for engineering ceramics.

[0002]

2. Description of the Related Art Silicon nitride is a material that is well balanced in wear resistance, high strength, fracture toughness, low specific gravity and low thermal expansion, and is an engineering ceramic for industrial machinery (hereinafter abbreviated as "Encera"). Although it is an ideal material for a member, its use range is limited because it is more expensive than alumina, which is a representative of Encera, and the appearance of inexpensive silicon nitride powder is desired.

Of the manufacturing processes of silicon nitride, the direct nitriding method of metal silicon is known as the cheapest manufacturing method, but the silicon nitride manufactured by this process has a high hardness and is therefore powdered. In order to do so, a multi-stage crushing process is required,
There was a problem of high cost. Further, there are problems in the multi-stage pulverization process such as mixing of impurities and introduction of a powder refining process.

[0004]

DISCLOSURE OF INVENTION Problems to be Solved by the Invention As a result of various studies to solve the above problems in the direct nitriding method of metal silicon and to produce silicon nitride having improved pulverizability,
The inventors have found that it is sufficient to add a silicon dioxide powder having a relatively large particle size to metal silicon and nitride it, and have completed the present invention.

[0005]

That is, according to the present invention, a mixed powder containing 1 to 20 parts by weight of silicon dioxide powder having an average particle diameter of 10 μm or more in terms of SiO ₂ is formed with respect to 100 parts by weight of metal silicon powder. And then heating it in a nitriding atmosphere.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below.

Conventionally, in the direct nitriding method of metallic silicon, silicon dioxide powder is added to the metallic silicon powder for the purpose of improving the sinterability of the silicon nitride powder and the high temperature strength characteristics of the sintered body. It is known that nitriding is performed in a nitriding atmosphere such as nitrogen added or in a reducing nitriding atmosphere containing nitrogen and ammonia or hydrogen, but the average particle diameter of the silicon dioxide powder used in each is the maximum. Was about 5 μm (specific surface area: about 10 m ² / g) (for example, JP-A-2-225306 and JP-A-2-248309).

On the other hand, the silicon dioxide powder used in the present invention has an average particle size of 10 μm or more, preferably JI.
The average particle size measured by S standard sieve is 100 to 500 μm
The feature is that the particles are significantly larger than conventional particles.

As in the present invention, by using a metal silicon powder to which a silicon dioxide powder having a large particle size is added as a raw material, a silicon nitride ingot containing a large amount of fine needle crystals or whiskers can be obtained. It can be produced, and when it is crushed into a roughly crushed / medium crushed product using an ordinary coarse crusher / medium crusher, the crushed product has a particle size of 0.5 mm or less and a specific surface area of 1.5 m ² / g or more. Of powder. The effect of such easy pulverization is that the specific surface area of the silicon nitride produced by adding the conventional silicon dioxide powder with a maximum of about 5 μm is about 1.0.
It is remarkably large as compared with about m ² / g.

That is, in the present invention, if the average particle size of the silicon dioxide powder is less than 10 μm, it is difficult to produce a crushable silicon nitride ingot containing a large amount of fine needle-like crystals or whiskers. . The upper limit of the average particle size of the silicon dioxide powder is not limited, but it is 1000 μm in consideration of the residual in silicon nitride.

The amount of the silicon dioxide powder added is 1 to 20 in terms of SiO _{2 with} respect to 100 parts by weight of the metal silicon powder.
Parts by weight It is preferably 1 to 10 parts by weight. If it is less than 1 part by weight, it is difficult to produce easily crushable silicon nitride, and if it exceeds 20 parts by weight, the amount of oxygen remaining in the silicon nitride cannot be ignored, and the strength of the silicon nitride sintered body is improved. It causes a significant decrease. Although it is desirable that the silicon dioxide powder has a high purity, it is also possible to use lozenge, silica stone, porcelain stone, etc. having a SiO ₂ purity of about 70% by weight or more.

On the other hand, the metal silicon powder used in the present invention is about 75 μm or less, preferably about 45 μm or less. As described above, the reason why the metal silicon having a relatively large grain size can be used in the present invention is that all the reactions of metal silicon and nitrogen are not solid-gas reaction (1), but some solid surface.・ Qi / Qi reaction after the Qi reaction (2)
It is greatly involved in the progress of nitriding in the formula. That is,
In the present invention, even if a coarse metal silicon powder having a size of about 75 μm or less is used, once a part of the surface undergoes solid-gas reaction to undergo nitriding, the density difference between the metal silicon and the silicon nitride causes a difference in the density of the metal silicon. The destruction phenomenon occurs and fine metal silicon is generated, and nitriding proceeds by gas-gas reaction.

To further explain, in the present invention, the reaction between the metal silicon powder and nitrogen is SiO 2 rather than solid-gas reaction.
The rate is controlled by the gas-gas reaction via (G), and the silicon nitride produced through this reaction has finer crystals and becomes easily crushable. The gas-gas reaction largely depends on the size of the added silicon dioxide powder.

Si (S) + 2/3 N ₂ (G) → 1/3 Si ₃ N ₄ (S) (1) SiO (G) + 2/3 N ₂ (G) → 1/3 Si ₃ N ₄ (S) + 1/2 O ₂ (2)

When silicon dioxide is produced by adding a catalyst such as an alkaline earth metal compound, its covalent bond may be broken and the melting temperature may be lowered. The lowering of the melting temperature becomes more remarkable as the particles of the silicon dioxide powder become finer, which is not preferable.

Next, the nitriding conditions of the mixed powder containing the metal silicon powder and the silicon dioxide powder will be described. In the reaction involving the present invention, the oxygen partial pressure in the atmosphere is low among the chemical species having Si as a nucleus (PO ₂ <10 ⁻⁵ atm).
In this case, the partial pressure of SiO (G) was the highest, followed by SiO ₂
(G), and the partial pressures of other chemical species are so small that they can be ignored. Further, the oxygen generated by the equation (2) oxidizes excess metal silicon present in the vicinity (see the equations (3) and (4)) and is circulated in the microscopic crystal field, so there is no problem.

Si (S) + 1/2 O ₂ (G) → SiO (S) → SiO (G) (3) Si (S) + O ₂ (G) → SiO ₂ (S) → reaction (5) (4)

The nitriding atmosphere used in the present invention is N ₂
Gas or a mixed gas of N ₂ gas and an inert gas (Ar, He, etc.). The N ₂ gas concentration is preferably 95% by volume or more.

The heating conditions are 1000 to 1450 ° C.
It is preferable that the temperature range is set to 10 ° C./hr or less. When the temperature rising rate exceeds 10 ° C./Hr, a local excess reaction occurs, and unnitrided metal silicon tends to remain due to melting of the metal silicon.

[0020]

The reason why the easily crushable silicon nitride is produced by the present invention is considered to be as follows. That is, when the silicon dioxide powder is a fine powder, the silicon dioxide is contained in a relatively low temperature range (a low temperature range that does not lead to the initiation of the nitriding reaction) before the SiO (G) partial pressure of the formula (2) is increased. Since it melts and fluidizes, it moves to the surface or the bottom of the mixed raw material and is concentrated, so that the effect of the silicon dioxide powder cannot be expected. On the other hand, as in the present invention,
When coarse-grained silicon dioxide powder is added, the reaction of the equation (2) is promoted by the equation (5), and the silicon dioxide is fluidized in a higher temperature range (around the temperature at which the nitriding reaction starts). Therefore, the partial pressure of SiO (G) is increased, and the nitriding reaction proceeds uniformly over the entire mixed raw material. As a result, easily crushable silicon nitride can be produced.

The easily crushable silicon nitride produced according to the present invention will be described in more detail. It is defined as an α-silicon nitride aggregate having a crystal form having a large surface area per unit mass. As shown in FIGS. 2 and 3, the aggregate does not include needle-like or columnar crystals having a large diameter, but includes fine whiskers or needle-like crystals having a diameter of 0.5 μm or less (see FIG. 1).

When the silicon nitride produced according to the present invention is crushed into a roughly crushed / medium crushed product using an ordinary coarse crusher / medium crusher, the specific surface area of the powder having a particle diameter of 0.5 mm is 1 in the crushed product. It becomes more than 0.5 m ² / g. This has a specific surface area of about 1.0 m ² / g in the case of conventional coarse and medium crushed silicon nitride.
It is extremely easy to grind as compared with the degree.

The α ratio of silicon nitride is higher than the α ratio (α ratio ≧
70%) is preferable. The reason is that the α phase dissolved in the sintering aid added at the time of sintering is increased, the β silicon nitride in which β columnar crystals with a high aspect ratio is developed is increased, and the strength development is improved. for that purpose,
(1) decrease the filling thickness of the raw material, (2) blend silicon nitride powder into the raw material, (3) control the reaction temperature and nitrogen gas concentration to keep the reaction rate below a certain level, (4) hydrogen in nitrogen gas Means such as mixing gas and / or ammonia gas, and (5) adding a catalyst such as fluoride to the raw material may be taken.

The silicon nitride produced as described above is subjected to dry or wet pulverization with a ball mill, a vibration mill, a jet mill, an attrition mill or the like after being subjected to a coarse crushing / medium crushing machine and adjusted to a desired particle size. It is put to practical use.

[0025]

The present invention will be described more specifically below with reference to examples and comparative examples.

Examples 1 to 8 Comparative Examples 1 to 7 Metallic silicon (Si purity 98% by weight, under 45 μm) 100
With respect to 100 parts by weight of the mixture, the additive powder shown in Table 1 was mixed with the parts by weight in the proportions shown in Table 2 (parts by weight for SiO ₂ equivalent to A to D, parts without conversion for E). 20 volume% polyvinyl alcohol aqueous solution 2
After adding 0 part by weight and kneading, mold it into a cylindrical specimen with a diameter of 150 mm and a height of 30 mm at a pressure of 20 kg / cm ^{2 and} dry it in the atmosphere at a temperature of about 200 for about 10 hours, and then nitrogen gas. 200 ℃ / Hr up to 1000 ℃ in an atmosphere
At a rate of 1000 ° C to 1450 ° C, 8 ° C / Hr
Heated at a rate of.

The obtained silicon nitride ingot was roughly crushed by using a jaw crusher and a double roll crusher, and then crushed by a mortar at 75 μm, and X-ray diffraction and oxygen content were measured. Table 2 shows the results. Also,
SEM photographs (magnification 3500 times) of the cross sections of the silicon nitrides obtained in Example 3, Comparative Example 3 and Comparative Example 7 are shown in FIGS. 1, 2 and 3, respectively.

[0028]

[Table 1]

[0029]

[Table 2]

Next, the coarsely ground product of silicon nitride obtained as described above was passed through a 0.5 mm sieve, and the specific surface area of the powder under the sieve was measured by the BET method (this was performed at a grinding time of 0).
The specific surface area over time is shown in Table 3). Next, 100 g of the sieved powder is a silicon nitride crushed ball (diameter 20 mm).
It grind | pulverized with a 1.2-liter filled 2 liter pot mill, the specific surface area after 2, 4 and 6 hours was measured, and the easy grindability was compared. Further, the particle size distribution of the powder after a crushing time of 6 hours was measured. Table 3 shows the results.

[0031]

[Table 3]

Further, the silicon nitride powders of Examples 2, 3, 4 and Comparative Example 7 obtained by ball milling for 6 hours were mixed with Y ₂ O ₃ powder having an average particle diameter of 1.3 μm and 1. 4μ
5% by weight and 4% by weight of Al ₂ O ₃ powder, respectively, and ethanol were further added, wet mixing was carried out with an alumina medium for 4 hours, and then dried to obtain 100 kg / cm ^2.
After molding into a mold of 6 × 10 × 60 mm with the molding pressure of 2.
CIP molding was performed at a molding pressure of 7 t / cm ² . These compacts were set in a carbon crucible and placed under a nitrogen gas atmosphere (1.
8 kg / cm ² ) and a temperature of 1750 ° C. for 8 hours to produce a sintered body. After grinding them, the relative density and room temperature 4-point bending strength were measured. The results are shown in Table 4.

[0033]

[Table 4]

The physical properties in this specification were measured as follows. (1) X-ray diffraction; "Geiger flux 2" manufactured by Rigaku Denki Co., Ltd.
013 "X-ray diffraction analyzer. α ratio = (α ₁ + α ₂ ) × 100 / (α ₁ + α ₂ + β ₁
+ Β ₂ ) However, α ₁ , α ₂ , β ₁ , and β ₂ are peak heights at predetermined positions obtained from the X-ray diffraction chart. α ₁ ; α-Si ₃ N ₄ (102) plane α ₂ ; α-Si ₃ N ₄ (210) plane β ₁ ; β-Si ₃ N ₄ (101) plane β ₂ ; β-Si ₃ N No. ₄ (210) face ND: Not detected from X-ray diffraction chart. W: A weak peak appears on the X-ray diffraction chart.

(2) Oxygen analysis; "EMG" manufactured by Horiba Ltd.
AA-2800 "oxygen / nitrogen analyzer. (3) Specific surface area; "QUN" manufactured by QUNTACHROME
TASORB Jr OS Jr-1 type ”device B
ET 1-point method. (4) Particle size distribution: "Microtrac SPA Model 7997" manufactured by Leeds & Northrup.
Laser diffraction type particle size distribution by the device (0.
2% sodium hexametaphosphate aqueous solution is used). (5) Relative density; by Archimedes method. (6) Bending strength at room temperature; "Autograph A" manufactured by Shimadzu Corporation
G-2000A "device.

[0036]

According to the present invention, since easily grindable silicon nitride can be produced by the direct nitriding method of metal silicon, an inexpensive silicon nitride powder can be provided.

The sintered body produced by using the silicon nitride obtained in the present invention has a relatively high relative density as compared with the sintered body produced by the conventional direct nitriding method silicon nitride powder of metal silicon. Both density and bending strength at room temperature are comparable.

[Brief description of drawings]

FIG. 1 is an SEM photograph (magnification: 3500 times) of a cross section of the silicon nitride manufactured in Example 3.

FIG. 2 is an SEM photograph (magnification: 3500 times) of a cross section of the silicon nitride manufactured in Comparative Example 3.

3 is a SEM photograph (magnification: 3500 times) of a cross section of the silicon nitride manufactured in Comparative Example 7. FIG.

Claims (1)

[Claims] 1. A mixed powder containing 1 to 20 parts by weight of a silicon dioxide powder having an average particle size of 10 μm or more in terms of SiO ₂ with respect to 100 parts by weight of a metal silicon powder is molded, and then the mixed powder is subjected to a nitriding atmosphere. A method for producing silicon nitride, which comprises heating.